1. Field of the Invention
This invention relates to electrochemical devices such as polymeric lithium secondary batteries and electric double-layer capacitors, and more particularly, to electrochemical devices having a failsafe mechanism against abnormal inflation and heat release.
2. Description of the Related Art
Various forms of batteries have been used in a wide variety of applications, mainly in electronic and automotive applications and as very large size ones for power storage. In such batteries, liquid electrolytes are often used. The replacement of liquid electrolytes by solid ones is expected to prevent liquid leakage and enable a sheet structure. Use of solid electrolytes is thus attractive for batteries of the next generation.
It is expected that if lithium ion secondary batteries which are frequently used in cellular phones and notebook computers can be fabricated to a small-size, sheet or laminate structure, their application will remarkably grow. For solid electrolytes, there have been proposed ceramic materials, polymeric materials and composite materials thereof. Among these, gel electrolytes in the form of polymer electrolytes plasticized with electrolytic solutions possess both a high electric conductivity inherent to liquid ones and a plastic character inherent to polymeric ones and are deemed potential in the future electrolyte development.
One advantage of batteries using solid electrolytes is an ability to form a thin large area structure, that is, a sheet structure. This will accelerate the further development of battery applications. The advantages of such sheet-shaped batteries are not obtained from the use of metallic casings as used in prior art cylindrical and rectangular batteries. Since the metallic casing accounts for a large proportion of the weight and thickness of the overall battery, the advantages of sheet-shaped batteries are offset. To take advantage of sheet-shaped batteries, it is requisite to use a lightweight laminate film as the casing or envelope.
When any anomaly occurs in a prior art battery using such a film as the envelope, the result is gas release and still worse, ignition, depending on the type of electrolyte. For example, a charger is designed to interrupt charging when the predetermined time or voltage is reached. If charging is not interrupted for some reason or other, the battery is over-charged in excess of its capacity. Further progress of over-charging can cause the electrolyte to be decomposed to give off gases to inflate the envelope, eventually leading to failure of the envelope or ignition.
Lithium ion batteries using metallic casings as the envelope are commercialized as having explosion-proof valves built therein. In the event of laminate film used as the envelope, it is difficult to install an explosion-proof valve and very difficult for such a valve to operate under the necessary pressure.
One technique of providing a laminate film with a valve is disclosed, for example, in JP-A 10-55792. The junction where opposed portions of laminate film are joined is provided with a region of a lower peel strength tapered from the inside to the outside. Usually, the junction is formed by bonding of a fusible resin. The region of lower peel strength is formed by introducing a non-fusible material (e.g., nickel foil) into the fusible resin, by effecting the bonding operation at a lower temperature than in the remaining area, or by leaving the region unbonded.
The lower peel strength region can function as a valve for venting gas when the internal pressure of the battery increases. For the region to exert the desired function, heating conditions during the bonding and/or the non-fusible material must be appropriately selected, which is not always easy in practice. In order that the envelope of laminate film prevent ingress of moisture and undesired contaminants from the exterior, the junction must have a seal width of at least about 4 mm. In the embodiment wherein the region of lower peel strength is defined in the junction by leaving the region unbonded during the bonding step, if the narrow portion of the junction disposed outside the lower peel strength region is 4 mm, the entire junction has a seal width far greater than 4 mm. This is disadvantageous from the energy density standpoint.
To avoid such an undesired phenomenon as rupture or ignition, batteries are normally provided with protective circuits. The protective circuits are often designed to shut off current flow when the predetermined voltage is reached.
However, assuming the situation where the protective circuit fails for some reason or other, a redundant protective means is often provided. Typical protective means are PTC elements and heat-sensitive protective components such as thermal fuses. The PTC elements are elements having a positive temperature coefficient, that is, elements which increase their resistance in response to a temperature rise. A sharp increase of resistance occurs above a certain temperature while the resistance change rate becomes of three figures, and even of six figures for some materials.
In general, chargers are designed to conduct a constant current flow until the predetermined voltage is reached and thereafter, control the current flow at the predetermined voltage. If the battery heats up by any anomaly during the charging process, the PTC element is heated to increase its resistance, thereby restraining the charging current flow for inhibiting further charging. The thermal fuse functions to shut off the charging current flow when the predetermined temperature is reached, interrupting the charging process. It is well known that lithium ion secondary batteries undergo thermal runaway above a critical temperature. The thermal runaway produces gases or further heat, inducing a failure or ignition of the battery. Therefore, the charging current flow must be reduced or shut off before the battery temperature reaches the critical level.
When a protective component is attached to the surface of an envelope, the maximum thickness of the battery including that protective component is often increased. Such a thickness increase is undesirable because electronic equipment such as cellular phones in which batteries are mounted are currently required to be smaller in size and thickness. It is thus desired to attach the protective component to the battery at a location aside from the maximum thickness.
However, the surface temperature of the battery is not always uniform. High heat radiating areas and low heat transfer areas have lower temperatures. Depending on the attachment location of the protective component, there is a possibility that the protective component operates at a lower temperature than other areas. This delays the time when the charging operation is interrupted, failing to prevent gas generation or ignition.
As mentioned above, even when the protective circuit is provided, there is a possibility that the protective circuit fails for some reason or other. Even in such a case, safety must be insured. When only a gas venting mechanism is added, a fire hazard can still be left because the charging operation continues even after the gas venting.
A first object of the invention is to provide an electrochemical device having reliability and safety owing to a simple venting means for venting gas in response to a rise of the internal pressure, the venting means serving as an explosion-proof valve and preventing ingress of moisture and contaminants.
A second object of the invention is to provide an electrochemical device comprising an envelope of flexible film having a protective component attached thereto, which is designed, without changing the maximum thickness of the device, to ensure that heat is transferred from the interior to the protective component whereby the device has improved safety.
A third object of the invention is to provide an electrochemical device, typically secondary battery, comprising an envelope of flexible film and a failsafe mechanism capable of preventing current flow on a gas generating accident.
In a first embodiment, the invention provides an electrochemical device comprising
an envelope having a sealable opening,
an electrochemical element having terminals, the electrochemical element being inserted in the envelope through the opening which is sealed to form a seal,
the envelope having a resin layer on its inner side adjacent the electrochemical element, and
a strip of a material different from the resin layer of the envelope, disposed in at least a portion of the seal, the strip serving as a means for relieving pressure within the envelope.
In a preferred embodiment, the sealable opening of the envelope is defined by opposed portions of the resin layer of the envelope, the strip is interposed at least in part between the opposed portions of the resin layer, and a seal is formed by joining the opposed portions of the resin layer together with the strip to provide the pressure relief means.
The strip is preferably made of a resin mixture of a first resin adhesive to the resin layer of the envelope and a second resin non-adhesive to the resin layer. Typically, the resin layer of the envelope is made of a first polyolefin resin, and the strip is disposed in contact with the resin layer and made of a resin mixture of the first polyolefin resin and a second polyolefin resin non-adhesive to the first polyolefin resin. Specifically, either one of the first and second polyolefin resins comprises polypropylene, and the other resin comprises polyethylene. Preferably, the resin mixture of which the strip is made contains a more amount of the second polyolefin resin than the first polyolefin resin. Specifically, the resin mixture of which the strip is made contains the first polyolefin resin and the second polyolefin resin in a weight ratio of from 40/60 to 15/85.
In a preferred embodiment, the terminals extend through the seal of the envelope, and the strip is disposed in the portion of the seal excluding the location of the terminals.
In a second embodiment, the invention provides an electrochemical device comprising
an envelope,
an electrochemical element received and sealed in the envelope, and
a heat-sensitive protective component for protecting the electrochemical element, the heat-sensitive protective component being attached to the envelope substantially outside the electrochemical element.
Provided that the electrochemical device has a maximum thickness D1 where the electrochemical element is received and a maximum thickness D2 where the heat-sensitive protective component lies, D1 is equal to or greater than D2.
Where the electrochemical device further comprises an internal electrode extending from the electrochemical element, a tab extending from the internal electrode, and an external electrode extending from the tab, the heat-sensitive protective component preferably lies at the location where any of the internal electrode, the tab and the external electrode is disposed.
Preferably the electrochemical device of the second embodiment has the pressure relief means of the first embodiment.
In a third embodiment, the invention provides an electrochemical device comprising
a flexible envelope,
an electrochemical element received and sealed in the envelope, the electrochemical element including internal electrodes and external electrodes electrically connected to the internal electrodes and extending outside the envelope, and
current shut-off means for shutting off either of the electrical connections between the internal electrodes and the external electrodes by detecting the stress created by inflation of the envelope.
Preferably the current shut-off means breaks the mechanical connection between the internal electrode and the external electrode by utilizing the stress. Preferably the internal electrode and the external electrode are attached to the envelope such that the connection between the internal electrode and the external electrode is preferentially broken by the stress. Provided that the internal electrode is attached to the envelope at a tensile strength A, the external electrode is attached to the envelope at a tensile strength B, and the internal electrode is connected to the external electrode at a tensile strength C, C is lower than A, and C is lower than B.
Preferably the electrochemical device of the third embodiment has the pressure relief means of the first embodiment and/or the heat sensitive protective component of the second embodiment.
In all the embodiments, the electrochemical device typically comprises a lithium secondary battery.